Mid-wheel power drive wheelchairs are known in the art and generally consist of a motorized base supporting a seat assembly. The motorized base generally comprises a central base frame equipped with a pair of drive wheels that are approximately positioned below the center of gravity of the vehicle when loaded with a user, a pair of front casters and a pair of rear, anti-tip casters, which prevent rearward tipping of the wheelchair. The pair of drive wheels are typically powered by individual compact electric motor and gearbox assemblies directly coupled to the inner side of each drive wheel, and a power battery conveniently tucked in a battery compartment generally occupying a central position between the drive wheels, just underlying the seat assembly.
The motorized base further comprises a support structure that interfaces the casters and drive wheels, including the motor and gearbox assemblies, with the rest of the wheelchair, namely the base frame, battery compartment and seat assembly. Typically, the support structure may represent a rigid support frame directly coupling the drive wheels and casters to the wheelchair, or a suspension system offering resilient suspension means between some or all of the wheels, and the rest of the wheelchair, thus allowing some degree of comfort to the user while traveling over an uneven or irregular ground surface. Typical examples of the prior art are U.S. Pat. No. 7,021,641B2, to Wu (2006), U.S. Pat. No. 6,073,951, to Jindra et al. (2000), U.S. Pat. No. 6,923,278B2, to Mulhern et al. (2005), and U.S. Pat. No. 6,070,898, to Dickie et al. (2000). The more complex support structures may further include pivotable front casters and drive wheels assemblies that allow a user to overcome abrupt obstacles such as a protuberant door step or the side edge of a sidewalk. In this regard, typical examples of the prior art are Pat. No. 2006/0249317A1, to Fought (2006), Pat. No. WO03/030800A1, to Molnar (2003), and Pat. No. EP1767178A1, to Hsu (2007).
While these motorized bases of the prior art can generally fulfill the main objective of enhancing the comfort of a user traveling aboard a powered wheelchair, they also offer one or more of the following disadvantages:
a) the generally limited flexibility of the suspension systems integrated in motorized bases of the prior art, particularly the suspension systems comprising resilient material such as rubber, instead of dynamic suspension strut, often result in a wheelchair experiencing uneven distribution of weight among its casters and drive wheels when engaging on an uneven or undulated ground surface. In some situations, one or more of the casters or drive wheels loose contact with the ground, thus resulting in a wheelchair having reduced stability;
b) moreover, while most motorized bases incorporates some form of suspension means applied to the front casters and/or drive wheels, the rear or anti-tip casters are generally rigidly attached to the rear end of the base frame and, thus, any small projections or irregularities encountered on the ground surface are directly transmitted to the seat and backrest assembly, adding to the discomfort of the user;
c) motorized bases equipped with pivotabie front casters and drive wheel assemblies may prove hazardous to use, particularly on an uneven or undulated ground surface since one or both front casters may inadvertently leave the ground due to a forced acceleration applied during a climbing operation;
d) generally, the inherent structure of the motorized bases does not allow for the custom positioning of the seat assembly and/or the battery compartment along the longitudinal axis of the central base frame, thus constraining the user to adopt a fixed position over a predefined center of gravity of the wheelchair.
Against this background, there exist a need for a new and improved wheelchair base. It is a general object of the present invention to provide a new and improved wheelchair base.